Slingsby T61 Motor Glider Project Diary

by Trevor Hewson


The Astrohog build went somewhat more quickly than expected so, after a few months break, I found myself in November 2009 staring at an empty building board. I put off the difficult question of what to build next by embarking on long overdue refurbishment of a couple of faithful old models (the Lazy Bee and P38) and was given an Autogyro kit for Christmas (see Rotorshape). So that got me through to the beginning of February but the UK 2010 Winter was one of those that seemed like it would never end so, even at that late stage, I found myself looking around for the next major build.

Eventually, I settled on the idea of a motor glider and before long found myself looking through Cliff Charlesworth's plans. The T61 seemed the right sort of size and so the plan, canopy and cowl were ordered and by the beginning of March I was in possession of a large pile of wood. I was somewhat alarmed to find that the wood weighed about 21lb which did make me question my 10 - 11lb all up weight target but then, I had over-ordered somewhat on the ply so, brushing aside such doubts, I set about clearing enough space on the bench to start.

March 2010 (More on March)

The wings on the T61 are huge and feature a spruce and ply box spar. This is not my preferred way of building since it can make it hard to ensure that the front and rear portions of the ribs come together on the spar at the appropriate angle to preserve the aerofoil section. The fact that the section is heavily undercambered only adds to the problem. New to me was the use of 1/64in ply rather than 1/16in balsa to sheet in the leading edge 'D' box. This of course means that any misalignment from rib to rib will result in a wavy surface to the wing, there being no scope at all for sanding out such irregularities. Furthermore the spar tapers in height and thickness and the front and rear ply skins reduce in thickness towards the tip. By now you will have gathered that I approached this build with some trepidation!

The spar itself was the obvious place to start.

Having made up the top and bottom spruce spars and cut out the ply facings, construction began. I used balsa spacers to try to manage the spar height so that it would match the ribs later (no rib cap strips are used so once again, there is no scope for fudging any height discrepancies).

Before fitting the second facing to the inner section of the spars, the brass joiner boxes had to be fitted. I did this with two pairs of balsa wedges to set the forward sweep and dihedral angles.

Once the spars were finished they were set aside to make way for a week of cutting out rib profiles. I cut all the ribs from 3/32 balsa on the basis that, where ply ribs are specified on the plan, I would apply thin doublers if necessary.

By the end of the month I had begun construction of the right hand wing by gluing the leading edge portions of the ribs in place, followed by a balsa false leading edge.

April 2010 (More on April)

Since the box spar is so torsionally rigid, I decided to fit the 1/64 leading edge sheeting before building the trailing edge section of the wing. The skins were cut out, sanded rough on the inside and smooth on the outside, then joined. What really took the time though was sanding the ribs and false leading edge to get everything lined up. I always thought that 1/16 balsa sheeting didn't allow much scope for sanding out any ripples but 1/64 ply doesn't allow anything at all!

Thesheets were first glued to the spar and weighted down, then pulled down to attach to the ribs and false leading edge. Once all the tape is removed, the appearance of the ply sheeting is quite pleasing.

The underside sheeting was applied in similar fashion then of course the process repeated for the other wing. The resulting D boxes are incredibly strong.

The process of setting out the trailing edge portion of the wing seemed unending - trimming the front ends to the correct angle, offering up the ribs to mark and notch them for sub spars, making holes for servo wiring tubes, and yet more notches and trimming to accommodate the spoiler. Eventually though, I could begin gluing them in place.

The plan shows a ply core slotted into the centre of the trailing edge of the ribs and balsa infill sections top and bottom. I didn't fancy that so instead inserted full depth 5mm balsa sections between the ribs, with a view to putting a ply skin on the top later.

By the end of the month I had got the top and bottom rear spars in place and also tackled that strange crank in the trailing edge at rib 3.

At this stage the right wing was set aside so that I could make a start on the left one - while I could still remember how I did it!

May 2010 (More on May)

With the second wing brought up to the same stage, the problem of photographing the work drove me outside. An ominous sign of the storage problems that lie ahead, I fear!

I was going to do the spoilers next and did make a start on one of them. However, this photo brought home to me just how vulnerable those aileron ribs are so I switched my attention to the trailing edge.

The fixed portion was tackled first with the result you see here. This was actually a departure from the arrangement shown on the plan - see detailed May diary entry for more details.

The aileron ribs were notched for the top and bottom spars (allowing for the subsequent facing) and the trailing edge was given the same treatment as the fixed section, with a selection of packing pieces used to set the washout.

On the construction front, that was just about it for May. Partly because we went out to Canada for two weeks to attend the Chilliwack fly-in - which also explains why this diary update is a bit late in appearing.

June 2010 (More on June)

Having completed the second aileron, the linkages were next. However, by opting for a semi concealed horn fixing, I made a bit of a meal of it but eventually got everything working okay.

I then spent a good bit of time improving the alignment of the underside of the ribs - some needed building up a bit and then sanding back to get them flush with the leading edge sheeting and rear spar. With as much as possible done on the underside, the wing was turned over and it was back to the spoilers. Even more fiddling here (see detailed June entry for more details) but again a reasonable end result.

Other than that, there's not a lot more to say this month. The diagonal spars were fitted to the root area, and the root bay of the underside was sheeted in with 1/64in ply (the top will be left open until the wing fixing is sorted out) .

The last job of the month was to cut the wing joiner bar to length so that the wings could be joined together. It came as something of a relief to see the forward sweep and dihedral angles for the first time - they both look to be about right.

The wings, including ailerons, spoilers and servos weigh 1lb 8oz each at this stage. They still have no root ribs, leading edges and tips of course.

July 2010 (more on July)
I promised myself that, come the end of June, I could put the wings aside for a bit and make a start on the fuselage, so the new month started with a bit of former building.

The formers are built up from a mixture of 1/16in ply and 1/16in balsa and hopefully represent a bit of a weight saving compared to the 4mm and 3mm ply specified on the plan.

The formers were then joined using basswood for the lower longerons. The root ribs and top spine are just dry fitted in this picture.

At this stage, I couldn't resist plugging the wings on to see how it looked.

'Big' was the answer!

Next came the upper longerons (basswood again) and the upper stringers. Spruce is specified for all longerons and stringers but I opted for balsa for the stringers, again in a quest to save a bit of weight at the tail end.

Once the vertical diagonals (?!) were in place, the root ribs could be fitted permanently, securing the main joiner box and incidence keeper tube in the process.

I did steam bend the basswood longerons to roughly the right shape before fitting them, so it was now time to fit the front two formers. Rather than move F1 forwards and re-calculate its shape, I positioned it as per plan and set about making a mounting box to support the motor.


With the motor temporarily fitted, the fit of the cowl could be checked - and for the first time it was possible to get some sort of idea of how the model might balance. It looks as if the 5s LiPo will need to go well forward so I'll be looking to save weight wherever possible at the tail end.


Before moving onto the tail surfaces though, I tidied up the wing roots, fitting the inner ply panels, leading edges and finally the 3mm ply root ribs and rubber band hooks.

I always like to get control linkages sorted out early, whilst access to the inside of the fuselage is still fairly open (you may have spotted a rudder servo temporarily fitted in the above photos). So, next step was to make a start on the tail surfaces.

And that's it for another month. Hopefully next month should see the tail surfaces finished and a bit more work on the fuselage. We shall see.

August 2010 (more on August)
The tailplane was covered in 1/16in balsa rather than the specified ply, in the quest to avoid the need for added noseweight, then it was on to the fin. I opted for a fixed fin, again to save the weight of the attachment mechanism, but retained the removeable tailplane, albeit with a modified fixing because of the need to remove it with the fin in situ.


The elevator and rudder both feature shrouded hinges which was a new adventure for me, particularly when it came to forming the leading edge of the rudder from 1/64 ply. More info as always on the detailed August diary page.


With the closed loop cables routed, I could move on to add the upper formers and stringers to the top of the fuselage, and then put it together to admire my work - and get a feel for the likely battery location.

Now that the top of the fuselage was built (with the significant exception of the cockpit and nose area) it was turned over in order to tackle the underside, and that monowheel undercarriage. The commercial wheel I had bought was rock hard and, although the plan warns against having the monowheel too soft, I felt that something more compliant was called for, eventually deciding to 'roll my own'.

The foam rubber discs were cut from a kneeling mat and eventually, when fitted to the hub from the rejected commercial wheel, gave quite a satisfactory result.

The underside was finished off with the wheel fairing and some infill sheeting, then more infill sheeting applied to the fuselage sides.


With the model balanced on its mainwheel, I could measure up for the outrigger legs, and then at last it was time to home in on that cockpit/ battery stowage area. I managed to find a pilot of a suitable size, so the challenge now is to work out how much of the cockpit interior can be fixed and what has to be removed for rigging the model and for battery insertion.


I was staggered to find that I took over 100 photos of this month's progress so, in order to stop this page from becoming totally bloated, an awful lot have been consigned to the August detail page. Don't forget to check it out here.

September 2010 (more on September)

The first task of the month was the construction of the canopy frame. After a lot of deliberation about ease of access, both for rigging the model and for battery changing, I decided to build the rear glazed section of the cabin onto the removeable canopy. This rules out the possibility of front-hinging the canopy as per the full size but, since the frame was a pretty fragile affair, I had already abandoned that idea anyway. The upside is that the connecting of the wing servo extension leads and fitting of the wing-retaining elastic bands should be a lot easier.

There was also much head-scratching about access to the battery position in the pilot's footwell. I did consider amputation but then spent quite a while trying to work out a way of fitting the battery through the passenger footwell under the pilot's feet. The breakthrough came when I spotted the air vent which sits on the top of the nose between the front of the canopy and the back edge of the cowl and realised that, by making this as a hatch, the battery could be dropped in vertically - with the added bonus that it keeps open the option of adding a passenger later - without the need for amputation. What's more, the canopy doesn't have to be removed for battery access.

The picture shows the partially built battery box, which is screwed in place. It will need some internal guide rails added later but for the moment there is still some leeway in the precise positioning of the battery.

I also made the basic seat bench at this point so that the pilot could be properly positioned.

At this stage the plans can't really help any more so they were rolled up and put away. With the top of the nose sheeted and the battery retention sorted, I assembled the model one last time before starting on the covering. Since everything was connected and functional, I couldn't resist the urge to taxi it around the garden for a bit!


Preparing the airframe for covering always involves a lot of work levellng joints etc., but eventually, I could make a start. The white is a cheap Oracover lookalike and seems to work very well. The red is Solarspan, bought along with Solarlac paint for the cowl in the hope of getting a good colour match.


I took regular breaks from the covering to re-glue the canopy to the frame. The basic problem is that the supplied canopy moulding is too wide and, being of quite heavy grade plastic, just pulls the glue joint slowly apart. To ease the stress of all this, I also made a start on upholstering the seats.


The seam in the cowling was filled with car bumper filler and passed to a fellow modeller, more handy with a spray gun than I. The luggage shelf area will be lined with the same 'Funky Foam' used on the seats (just trial-fitted in this picture).


By the end of the month the covering was finished apart from the area immediately behind the cowl. Once that is done and the rear cockpit glazing fitted there is not a lot standing in the way of the maiden flight.

Of course the pilot still has no seat belts, instruments or controls and the registration markings are missing but these are not quite as flight-critical on a model as on the full size aircraft!

As usual, more details and pictures on the September Diary Page.

October 2010 (more on October)

The pilot and passenger harnesses were made from black ribbon, servo extension lead housings being used for the buckles and the adjuster slides cut from plasticard. The straps are all that hold the pilot in place, so they have to be functional.

The fuel gauge is a simple sight tube, the tank being mounted behind the cockpit. With a photo of the full size gauge for reference, the gauge was drawn up on the computer and printed out.

Glazing the rear canopy was the last flight-critical job so, with that done, the model was put together one last time for final balance checks, a power run-up and another taxiing session in the garden. All seemed in order so it was now just a question of waiting for the weather.

While awaiting suitable conditions for the maiden flight, the pilot was equipped with a spoiler operating lever and control column. Still no instruments at this stage though!

It wasn't long before I got my chance. A fairly dull, cold day but dry and with light winds. Once the model was assembled there were a few more photographs and a range check.

Then, after a final control check, the model was lined up for take off. It was soon clear that there was plenty of power and the T61 lifted off without drama.

Conditions were poor for photography but, thanks to Tim, I do have a record of the maiden flight, which was pleasantly uneventful.

The spoilers are quite powerful and the model does pitch down when they are deployed. Correcting for this resulted in a rather lumpy final approach but the landing was safe enough. I have tried mixing a bit of up elevator with the spoilers but the model does have to be allowed to pick up a bit of speed to compensate for the lift shed by the spoilers so some initial downward pitch is inevitable.

I have also reduced the elevator travel and subsequent landings have been a lot easier.

Over the next two outings, we did manage to get some better flying shots and a short video clip which you can see elsewhere in these pages but of course, the T61 was far from being finished. On the RCGroups thread, people had teased me about the pilot's grey hair and staring eyes so, whilst I made him some sunglasses out of a red paper clip, Mary ran up a matching red hat. Of course those same people now teased me about giving him a pink hat but at least now, if he starts getting bounced around a bit, he won't scratch the canopy!

At this stage, after being thwarted a few times by weather or diary clashes, I finally got to meet up with the full size G-BUIH and its pilot, Lee. I have published the photos (and a video of a take-off) on a separate page (see here)

From the photos taken at this meeting, I felt able to press ahead with a few more details, starting with that missing instrument panel. I had bought a set of 1/4 scale instrument decals but opted instead to Photoshop a few of my photos together, and built a throttle plunger out of bits of plastic and a couple of electrical grommets.

Carrying on the electrical recycling theme, the live wire from a piece of 30a cable was used for the cockpit access steps, and cable was also used for the bent and squashed tubing of the canopy jettison bar.

With a full complement of spoiler levers etc., the cockpit is looking busy enough for me to call a halt.

So, just a few more external bits, such as the fuel filler (made from balsa) and those outrigger legs (sleeved with white heatshrink), and I reckon I am ready to declare this model finished.

Well, the canopy for the next project arrived in the post this morning and the wood came yesterday so, after eight months, I'm ready for a change!

Don't forget to check out the other pages linked below. I hope to add some more flying shots to the Slideshow in due course, meanwhile there are quite a few on the October diary page.

Also, if you want more detail, remember to click on the monthly diary headings or, to see some of the inputs from other modellers who helped me along the way, head over to the build thread on RCGroups.